INSIDIOUS Synonyms: 100 Similar and Opposite Words

insidious

adjective

Definition of insidious

as in treacherous

formal causing harm in a way that is gradual or not easily noticed Most people with this insidious disease have no idea that they are infected. an insidious enemy

Related Words

Relevance

Dissimilar Words

Example Sentences

Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.

Recent Examples of insidious Growing up in Beverly, Frates – the warrior against ALS who succumbed to the insidious disease in 2019 – was a huge fan of the Bruins’ star and had a poster of the defenseman in his room. — Steve Conroy, Boston Herald, 6 Sep. 2026 But the insidious harm that moves in the other direction — from illness, through a patient, into the person treating her — has no forum at all. — Jennifer Adaeze Okwerekwu, STAT, 1 Sep. 2026 When his insidious nature is revealed, the change in tone and his face gone malformed, the scene feels dynamic in a way that doesn’t require gore or intricate special effects. — Literary Hub, 28 Aug. 2026 The broader opportunity costs of the United States’ Middle Eastern follies are even more insidious. — Emma Ashford, Foreign Affairs, 12 Aug. 2026 See All Example Sentences for insidious

Recent Examples of Synonyms for insidious

Adjective

The Sacramento State junior quarterback has had his moments in the Hornets’ 1-3 start in their first season navigating the tricky and treacherous waters of the NCAA Division I FBS high seas.

— Joe Davidson, Sacbee.com, 24 Sep. 2026
Heavy downpours drenched parts of northern Broward County on Thursday, causing treacherous driving conditions as rain flooded streets and roads.

— Joan Murray, CBS News, 18 Sep. 2026

Definition of treacherous

Adjective

The comic tension embodies Cassavetes’ approach to the entire film, which involved some not-so-subtle manipulation of his actors behind the scenes.

— Jim Hemphill, IndieWire, 23 Sep. 2026
And just like her clothes and accessories serve as a subtle nod to the '90s, so does her beauty look.

— Marci Robin, Allure, 23 Sep. 2026

Definition of subtle

Adjective

That’s why Meta has agreed to pay up to eighteen billion dollars as part of a settlement over predatory and deceptive practices, and why Anthropic is paying writers and publishers (admittedly, pennies) in a copyright settlement.

— Jill Lepore, New Yorker, 21 Sep. 2026
Some states have enacted laws requiring disclosures or restricting deceptive political deepfakes near elections.

— Reed Williams, AJC.com, 21 Sep. 2026

Definition of deceptive

Adjective

In the wake of the 2020 election, false accusations led to death threats and upended the lives of two Fulton County election workers.

— Caleb Groves, AJC.com, 23 Sep. 2026
Election law generally requires registration in every state except North Dakota, while federal law prohibits false registration and other forms of election fraud.

— Los Angeles Times, Los Angeles Times, 23 Sep. 2026

Definition of false

Adjective

The final answer may cite real sources and still be misleading.

— Patrick Dajos, Forbes.com, 23 Sep. 2026
Providing false or misleading information to law enforcement undermines those efforts and unnecessarily diverts officers and resources away from members of our community who may need immediate police assistance.

— Brie Stimson, FOXNews.com, 19 Sep. 2026

Definition of misleading

Adjective

Bogus texts, mystery calls, deceitful emails all aimed at stealing your personal information and your money are relentless.

— Donna Vickroy, Chicago Tribune, 13 Aug. 2026
The mutual trust and goodwill that xenia sought to cultivate has been eroded by this one deceitful act, creating a world that is defined by this moral violation.

— Brianna Zigler, Entertainment Weekly, 19 July 2026

Definition of deceitful

Adjective

The appropriate level of human involvement depends on financial exposure, customer impact, regulatory requirements, security implications, reversibility and the consequences of an incorrect decision.

— Jayasri Ranganathan, Forbes.com, 29 Sep. 2026
But predictions that the Islamic Republic would collapse proved incorrect.

— Ali Vaez, Foreign Affairs, 29 Sep. 2026

Definition of incorrect

Browse Nearby Words

inside tracks

insidious

insidiously

See all Nearby Words

Articles Related to insidious

Cite this Entry

Style

“Insidious.” Merriam-Webster.com Thesaurus, Merriam-Webster, https://www.merriam-webster.com/thesaurus/insidious. Accessed 1 Oct. 2026.

Copy Citation

Synonyms of insidious

adjective

Example Sentences

Browse Nearby Words

Words of the Year: A Decade in Review

Red Carpet Words: Our Top 7...

2014 Word of the Year: Culture

Cite this Entry

More from Merriam-Webster

Word of the Day

slew

Popular in Grammar & Usage

Do You Need a New Language Peeve?

More Commonly Mispronounced Words

Merriam-Webster’s Great Big List of Words You Love to Hate

'A' or 'an'? What about before 'h'? An Indefinite Article Guide

Why is '-ed' sometimes pronounced at the end of a word?

Popular in Wordplay

7 Great Ways to Say 'Great'

The Longest Long Words List

9 Words That Sound Like Insults But Aren't

12 Lovely and Unusual Words for the Natural World

Why do we 'bury the lede?'

Popular

Do You Need a New Language Peeve?

7 Great Ways to Say 'Great'

The Longest Long Words List

Games & Quizzes